Our laboratory is engaged in the determination, primarily by x-ray crystallographic technique, of the structure and function of sugar-binding proteins for active transport and chemotaxis in bacteria. A complete molecular model of the L-arabinose-binding protein has been accomplished by fitting the sequence for 306 residues to a 2.4 angstrom units electron density map. The structure is currently being refined at 1.9 angstrom units resolution. The stereochemistry of the mode of binding of sugar substrates to the protein has been deduced. All the hydroxyls of L-arabinose are hydrogen-bonded to residues. The sugar bound in the cleft between the two domains is inaccessible to the solvent, indicating a conformational change. The 4.1 angstrom units structure of D-galactose-binding protein has been determined, it looks like L-arabinose-binding protein.